Newly synthesized eukaryotic mRNA molecules, also known as primary transcripts or pre-mRNA, made in the nucleus, are processed before or during transport to the cytoplasm for translation. Processing of the pre-mRNAs includes addition of a 5′ methylated cap and an approximately 200-250 base poly(A) tail to the 3′ end of the transcript.
The next step in mRNA processing is splicing of the pre-mRNA, which occurs in the maturation of 90-95% of mammalian mRNAs. Introns (or intervening sequences) are regions of a primary transcript (or the DNA encoding it) that are not included in the coding sequence of the mature mRNA. Exons are regions of a primary transcript that remain in the mature mRNA when it reaches the cytoplasm. The exons are spliced together to form the mature mRNA sequence. Splice junctions are also referred to as splice sites with the 5′ side of the junction often called the “5′ splice site,” or “splice donor site” and the 3′ side the “3′ splice site” or “splice acceptor site.” In splicing, the 3′ end of an upstream exon is joined to the 5′ end of the downstream exon. Thus the unspliced RNA (or pre-mRNA) has an exon/intron junction at the 5′ end of an intron and an intron/exon junction at the 3′ end of an intron. After the intron is removed, the exons are contiguous at what is sometimes referred to as the exon/exon junction or boundary in the mature mRNA. Cryptic splice sites are those which are less often used but may be used when the usual splice site is blocked or unavailable. Alternative splicing, defined as the splicing together of different combinations of exons, often results in multiple mRNA transcripts from a single gene.
Up to 50% of human genetic diseases resulting from a point mutation are caused by aberrant splicing. Such point mutations can either disrupt a current splice site or create a new splice site, resulting in mRNA transcripts comprised of a different combination of exons or with deletions in exons. Point mutations also can result in activation of a cryptic splice site or disrupt regulatory cis elements (i.e. splicing enhancers or silencers) (Cartegni et al., Nat. Rev. Genet., 2002, 3, 285-298; Krawczak et al., Hum. Genet., 1992, 90, 41-54).
Antisense oligonucleotides have been used to target mutations that lead to aberrant splicing in several genetic diseases in order to redirect splicing to give a desired splice product (Kole, Acta Biochimica Polonica, 1997, 44, 231-238). Phosphorothioate 2′-O-methyl oligoribonucleotides have been used to target the aberrant 5′ splice site of the mutant β-globin gene found in patients with β-thalassemia, a genetic blood disorder. Aberrant splicing of mutant β-globin mRNA was blocked and normal splicing was restored in vitro in vector constructs containing thalassemic human β-globin pre-mRNAs using 2′-O-methyl-ribo-oligonucleotides targeted to the branch point sequence in the first intron of the mutant human β-globin pre mRNAs (Dominski and Kole, Proc. Natl. Acad. Sci. USA, 1993, 90, 8673-8677). Oligonucleotides targeted to the aberrant β-globin splice site suppressed aberrant splicing and at least partially restored correct splicing in HeLa cells expressing the mutant transcript (Sierakowska et al., Nucleosides & Nucleotides, 1997, 16,1173-1182; Sierakowska et al., Proc. Natl. Acad. Sci. USA, 1996, 93, 12840-44). U.S. Pat. No. 5,627,274 and WO 94/26887 disclose compositions and methods for combating aberrant splicing in a pre-mRNA molecule containing a mutation using antisense oligonucleotides which do not activate RNAse H.
Modulation of mutant dystrophin splicing with 2′-O-methyl oligoribonucleotides has been reported both in vitro and in vivo. In dystrophin Kobe, a 52-base pair deletion mutation causes exon 19 to be skipped during splicing. An in vitro minigene splicing system was used to show that a 31-mer 2′-O-methyl oligoribonucleotide complementary to the 5′ half of the deleted sequence in dystrophin Kobe exon 19 inhibited splicing of wild-type pre-mRNA. The same oligonucleotide was used to induce exon skipping from the native dystrophin gene transcript in human cultured lymphoblastoid cells (Takeshima et al., J. Clin. Invest., 1995, 95, 515-520).
Dunckley et al. (Nucleosides & Nucleotides, 1997, 16, 1665-1668) describes in vitro constructs for analysis of splicing around exon 23 of mutated dystrophin in the mdx mouse mutant, a model for Duchenne muscular dystrophy. 2′-O-methyl oligoribonucleotides were used to correct dystrophin deficiency in myoblasts from the mdx mouse. An antisense oligonucleotide targeted to the 3′ splice site of murine dystrophin intron 22 caused skipping of the mutant exon and created a novel in-frame dystrophin transcript with a novel internal deletion. This mutated dystrophin was expressed in 1-2% of antisense treated mdx myotubes. The use of other oligonucleotide modifications, such as 2′-O-methoxyethyl phosphodiesters, is disclosed (Dunckley et al. Human Mol. Genetics, 1998, 5, 1083-90).
Phosphorothioate oligodeoxynucleotides have been used to selectively suppress the expression of a mutant α2(I) collagen allele in fibroblasts from a patient with osteogenesis imperfecta, in which a point mutation in the splice donor site produces mRNA with exon 16 deleted. The oligonucleotides were targeted either to the point mutation in the pre-mRNA or to the defectively spliced transcript. In both cases mutant mRNA was decreased by half with the normal transcript decreased by 20% (Wang and Marini, J. Clin Invest., 1996, 97, 448-454).
Antisense compounds have been used to block cryptic splice sites to restore normal splicing of HBB (β-globin) and CFTR genes in cell lines derived from β-thalassemia or cystic fibrosis patients, respectively (Lacerra et al., Proc. Natl. Acad. Sci. USA, 2000, 97, 9591-9596; Friedman et al., J. Biol. Chem., 1999, 274, 36193-36199). Antisense compounds have also been used to alter the ratio of the long and short forms of Bcl-x pre-mRNA (U.S. Pat. No. 6,172,216; U.S. Pat. No. 6,214,986; Taylor et al., Nat. Biotechnol. 1999, 17, 1097-1100) or to force skipping of specific exons containing premature termination codons (Wilton et al., Neuromuscul. Disord., 1999, 9, 330-338).
Kole et al. (WO 94/26887 and U.S. Pat. Nos. 5,627,274; 5,916,808; 5,976,879; and 5,665,593) disclose methods of combating aberrant splicing using modified antisense oligonucleotides which do not activate RNase H.
U.S. Pre-Grant Publications 2003-0114411 and 2003-0036519 discuss antisense oligonucleotides conjugated to a nuclear localization element for use in upregulating expression of a cellular protein, which is encoded by a DNA comprising a mutation which results in aberrant splicing of the pre-mRNA.
A method of controlling the behavior of a cell through modulation of the processing of an mRNA target by contacting the cell with an antisense compound acting via a non-cleavage event is disclosed in U.S. Pat. No. 6,210,892 and U.S. Pre-Grant Publication 2002-0049173.
PCT Publication WO 02/38738 and U.S. Pre-Grant Publication 2005-0054836 disclose chimeric molecules comprising a base-pairing segment which specifically binds to a single-stranded target nucleic acid, and a moiety which modulates splicing or translation. Further disclosed are methods of using the chimeric molecules for modulating splicing or translation of a target nucleic acid molecule.
U.S. Pre-Grant Publication 2005-0074801 discloses chimeric oligomeric compounds with regions of nucleosides that are DNA-like and regions of nucleosides that are RNA-like.
Although a number of antisense compounds that are capable of modulating splicing of a target gene in vitro have been reported, there remains a need to identify compounds suitable for therapeutic use in vivo. In order for an antisense oligonucleotide to achieve therapeutic success, oligonucleotide chemistry must allow for adequate cellular uptake (Kurreck, J. (2003) Eur. J. Biochem. 270:1628-1644). Splicing oligonucleotides have traditionally been comprised of uniform modifications that render the oligonucleotide RNA-like, and thus resistant to cleavage by RNase H, which is critical to achieve modulation of splicing. Provided herein are antisense compounds for modulation of splicing. The disclosed compounds are chimeric, with regions of RNA-like and DNA-like chemistry. Despite regions of DNA-like chemistry, the chimeric compounds are RNase H-resistant and effectively modulate splicing of target mRNA in vitro and in vivo. Furthermore, the disclosed chimeric oligomeric compounds show enhanced cellular uptake and greater pharmacologic activity compared with uniformly modified oligonucleotides.